Condensation assessment: which approach?

 

A major challenge for any specifier in the new refurbishment market is to assess the appropriateness of one buildup and specification over another. Without the ability (resources or time) to do primary research and without sophisticated simulation software architects and builders generally rely on data supplied by standards authorities and suppliers, backed up by codes. But is that always good enough? What’s the quality of the information?

In relation to condensation risk, the relevant document of a standards authority (e.g. an agrément certificate) issued anytime in the last seven years is likely to state something like this:

 

The design will include for… minimising risk of condensation in accordance with the recommendations of “BS 5250: 2002 – Code of Practice for Control of Condensation in Buildings”.

 

In the case of a supplier the relevant portion of the brochure could state that their:

(…) Technical Service Department can provide a condensation risk analysis of your proposed design. Alternatively, the designer can undertake an independent assessment by following the procedures set out in BS 5250: 2002 (…)

 

Or it could state:

When insulating buildings the recommendations of “I.S. EN 13788: 2001 – Hygrothermal Performance of Building Components and Building Elements – Internal Surface Temperature to avoid Critical Surface Humidity and Interstitial Condensation – Calculation Methods” should be followed to minimise the risk of condensation within the building elements and structures.

 

BS 5250 defines the issues, parameters and terms in relation to condensation in buildings and shows some calculations. IS EN 13788 ‘gives calculation methods for the internal surface temperature (…) below which mould growth is likely, given the internal temperature and relative humidity’ and ‘the risk of interstitial condensation due to water vapour diffusion’. It appears that the calculation methods usually selected for use from these documents are:

1.     The establishment of the temperature factor (fRsi) to determine likelihood of surface condensation as set out in Annex D.2 of BS 5250 (also found in Appendix D of TGD L), and

2.     The Glaser or dewpoint method for assessing ‘the risk of interstitial condensation due to water vapour diffusion’ in Section 6 of IS EN 13788. The Glaser method itself has been used for decades and is a well-established tool in the Construction Industry (see Figure below).

 

IS EN 13788 also refers briefly in Annex F to more advanced calculation models, i.e. computer models and a version of the Glaser method which makes an allowance for moisture redistribution in the liquid phase (i.e. capillary action). It is unlikely that a supplier making reference to an analysis under IS EN 13788 (as quoted above) would be referring to use of the Annex F models as they are barely sketched out in that document. This author would be very interested to hear from insulation suppliers that use a wider range of assessment methods than the two listed above (1, 2).

 

The impact of further growth in understanding of building physics, increased processing power and the development of sophisticated simulations software since those documents were published require a re-appraisal of how and when we use them. The creators of IS EN 13788 were very clear on the limitations of the calculation methods they set out and knew that numerical simulation would allow far greater accuracy once the input data was better known:

Therefore this standard lays down simplified calculation methods, based on experience and commonly accepted knowledge. The standardisation of these calculation methods does not exclude use of more advanced methods. (…) The [Glaser] method should be regarded as an assessment rather than as an accurate prediction tool. It does not provide an accurate prediction of moisture conditions within the structure under service conditions (…)

The ‘computer models’ and ‘accurate prediction tools’ are now available for use by specifiers and suppliers: IS EN 15026 (2007) is the ‘protocol’ they adhere to. Its Introduction gives a useful summary of the differences between a condensation prediction under that standard and an assessment using the Glaser method:

This standard [IS EN 15026] defines the practical application of hygrothermal simulation software used to predict one-dimensional transient heat and moisture transfer in multi-layer building envelope components subjected to non-steady climate conditions on either side… While the Glaser method considers only steady-state conduction of heat and vapour diffusion, the transient models covered in this standard take account of heat and moisture storage, latent heat effects, and liquid and convective transport under realistic boundary and initial conditions. The application of such models has become widely used in building practice in recent years, resulting in a significant improvement in the accuracy and reproducibility of hygrothermal simulation.

 

One-dimensional numerical simulation is becoming and will then remain the main tool for day to day assessment. WUFI Pro from the Fraunhofer Institute is a leading example of this simulation software. The kind of questions it allows to be studied and the degree of accuracy it delivers could only have been dreamed of by Mr. Glaser when he first formulated his method in 1959.

 

Date

Monday, August 30, 2010